The electron mobility of a polycrystal silicon thin film transistor is greater by a factor of 100 than that of an amorphous silicon thin film transistor. The use of polycrystal silicon thin film transistors allows the miniaturization of elements and the denser mounting of driving circuits on one substrate. In the field of liquid crystal display devices, polycrystal silicon thin film transistors are recently used in thin film transistor arrays with built-in driving circuits. These thin film transistor arrays with built-in driving circuits have been made possible by the development of technology to manufacture arrays on glass substrates which can be easily enlarged.
To form polycrystal thin film transistors at low temperatures, the development of a method for activating the dopant implanted into the polycrystal silicon thin film at low temperatures is important as well as technology to form polycrystal silicon thin film at low temperatures. Low temperature crystallization using excimer laser annealing is often employed to form good polycrystal silicon thin films on large substrates at low temperatures.
For example, IEEE Electron Device Letters, Vol. EDL-7, No. 5, May 1986, pp. 276-278, discloses technology related to excimer laser annealing. In general, thermal annealing is used for activation, but the activation rate significantly drops as a result of reducing the processing temperature.
Rapid thermal annealing (RTA) and excimer laser activation are proposed as methods for improving the dopant activation rate at low temperatures to counteract the above disadvantage. SID97 M/52: Recent Advances in Rapid Thermal Processing of Polysilicon TFT LCDs discloses RTA activation, and the Extended Abstract of the 18th (1986) International Conference on Solid State Devices and Materials, pp. 225-228, discloses excimer laser activation.
FIGS. 3A to 3D show process flow charts describing a conventional method of manufacturing polysilicon thin film transistors for the active matrix arrays used in liquid crystal display devices. As shown in FIG. 3A, a silicon oxide film which becomes a buffer layer 12 is formed on a transparent glass substrate 11 using the plasma CVD method. Amorphous silicon (a-Si) film is then deposited using the plasma CVD method without exposing the substrate 11, on which the buffer layer 12 is formed, to air.
Next, a thermal treatment is applied to reduce the hydrogen in the a-Si film. The a-Si film is polycrystallized by excimer laser annealing to form a poly-Si film 13a. Finally, the poly-Si film 13a is processed into the size and shape required for a TFT.
Next, a silicon oxide film which becomes a gate insulation film 14 is formed. A gate electrode 15 typically made of Al alloy is formed and dopant is implanted to form a Lightly Doped Drain (LDD) region 13b in the thin film transistor as shown by an arrow 100 in FIG. 3A. As shown in FIG. 3B, a mask for implanting dopant into the source and drain regions is then formed using a photo resist 25 in a manner to cover the LDD region 13b of the thin film transistor. A large quantity of phosphorus ion, the dopant, is implanted into the source region 21 and drain region 22 by ion implantation, as shown by an arrow 100 in FIG. 3B. The source region 21 and drain region 22 which have high concentrations of dopant are called a SD region 13C.
Since the implanted dopant is electrically inactive, excimer laser light is applied, as shown by an arrow 101 in FIG. 3C, to activate it.
Then, as shown in FIG. 3D, a silicon oxide film which becomes an interlayer insulation film 16 is formed and contact holes 17a and 17b are opened on the insulation film in the source region 21 and drain region 22. A layered film of Ti and Al is formed and processed to form SD wirings 18a and 18b.
Finally, a protective insulation film 23 made of silicon nitride is formed, and annealed in a hydrogen atmosphere. Hydrogen annealing fills the empty ionic bonds in the polycrystal silicon thin film with hydrogen, enabling the characteristics of the thin film transistor to be improved.
However, the conventional method of activation using an excimer laser causes a high degree of thermal damage to the gate electrode 15. More specifically, as shown in FIG. 3C, an irradiated excimer laser light is applied to and absorbed by the polycrystal silicon through the gate insulation film 14 at the source region 21 and drain region 22 of the thin film transistor. The laser light applied to the gate electrode 15 region is also directly absorbed by the gate metal, causing the gate electrode's temperature to rise. If metals with high melting points such as W, Mo, and Cr are used for the gate electrode 15, cracks or peeling of the gate electrode 15 may occur as a result of thermal shock due to laser irradiation. If Al alloy is used for the gate electrode 15, quality problems such as an increase in hillocks may occur. Hillocks are the phenomenon whereby the material surface becomes bumpy as a result of temperature rise.
The present invention provides a thin film transistor manufacturing method and thin film transistor which reduces the thermal damage to gate electrodes caused by laser irradiation during the manufacture of thin film transistors which includes the process of dopant activation by laser irradiation.